In the field of a magnetic disc, a 2 MB MF-2HD floppy disc using Co-modified iron oxide has been generally loaded in a personal computer. However, along with the increase in the amount of data to be dealt with, the capacity thereof has become insufficient and the increase of the capacity of the floppy disc has been demanded.
Also, in the field of a magnetic tape, with the prevalence of an office computer, such as a minicomputer, a personal computer and a work station, a magnetic tape for recording computer data as external storage medium (a so-called backup tape) has been vigorously studied. For the realization of the magnetic tape for such a use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing ability (e.g., information throughput).
Magnetic layers comprising an iron oxide, a Co-modified iron oxide, CrO.sub.2, a ferromagnetic metal powder, or a hexagonal ferrite powder dispersed in a binder, which are coated on a nonmagnetic support, have been conventionally widely used in magnetic recording media. Ferromagnetic metal powders and hexagonal ferrite powders among these have been known to have excellent high density recording characteristics.
In the case of a disc, as high capacity discs using ferromagnetic metal powders which are excellent in high density recording characteristics, there are 10 MB MF-2TD and 21 MB MF-2SD, and as high capacity discs using hexagonal ferrite, there are 4 MB MF-2ED and 21 MB Floptical, however, any of these are not satisfactory with respect to capacities and properties. As is the circumstance, various attempts have been made to improve high density recording characteristics. Some examples thereof are described below.
For improving characteristics of a disc-like magnetic recording medium, JP-A-64-84418 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes the use of a vinyl chloride resin having an acidic group, an epoxy group and a hydroxyl group, JP-B-3-12374 (the term "JP-B" as used herein means an "examined Japanese patent publication") proposes the use of a metal powder having a coercive force Hc of 1,000 Oe or more and a specific surface area of from 25 to 70 m.sup.2 /g, and JP-B-6-28106 proposes to regulate the specific surface area and magnetic susceptibility of magnetic powders and contain an abrasive.
For improving the durability of a disc-like magnetic recording medium, JP-B-7-85304 proposes the use of a fatty acid ester having an unsaturated fatty acid ester and an ether bond, JP-B-7-70045 proposes the use of a fatty acid ester having a branched fatty acid ester and an ether bond, JP-A-54-124716 proposes the use of a nonmagnetic powder having a Mohs' hardness of 6 or more and a higher fatty acid ester, JP-B-7-89407 proposes to regulate the volume of voids containing a lubricant and the surface roughness to from 0.005 to 0.025 .mu.m, JP-A-61-294637 proposes the use of a fatty acid ester having a low melting point and a fatty acid ester having a high melting point, JP-B-7-36216 proposes the use an abrasive having a particle size of from 1/4 to 3/4 of the magnetic layer thickness and a fatty acid ester having a low melting point, and JP-A-3-203018 proposes the use of a metallic magnetic powder containing Al and a chromium oxide.
As the constitution of a disc-like magnetic recording medium having a nonmagnetic lower layer and an intermediate layer, JP-A-3-120613 proposes the constitution comprising an electrically conductive layer and a magnetic layer containing a metal powder, JP-A-6-290446 proposes the constitution comprising a magnetic layer having a thickness of 1 .mu.m or less and a nonmagnetic layer, JP-A-62-159337 proposes the constitution comprising an intermediate layer comprising a carbon and a magnetic layer containing a lubricant, and JP-A-5-290358 proposes the constitution comprising a nonmagnetic layer in which the carbon size is regulated.
On the other hand, a disc-like magnetic recording medium comprising a thin magnetic layer and a functional nonmagnetic layer has been developed in recent years and floppy discs of the class with the capacity of 100 MB are now on the market. As floppy discs showing these characteristics, JP-A-5-109061 proposes the constitution comprising a magnetic layer having Hc of 1,400 Oe or more and a thickness of 0.5 .mu.m or less and a nonmagnetic layer containing electrically conductive particles, JP-A-5-197946 proposes the constitution comprising abrasives having particle sizes larger than the thickness of the magnetic layer, JP-A-5-290354 proposes the constitution comprising a magnetic layer having the thickness of 0.5 .mu.m or less and the fluctuation of the thickness of within .+-.15%, in which the surface electric resistance is regulated, and JP-A-6-68453 proposes the constitution in which two kinds of abrasives having different particle sizes are contained and the amount of the abrasives on the surface is regulated.
Further, in the field of a tape-like magnetic recording medium, with the prevalence of an office computer, such as a minicomputer and a personal computer, a magnetic tape for recording computer data as external storage medium (a so-called backup tape) has been vigorously studied. For the realization of the magnetic tape for such use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing capability. In addition, the use in various environmental conditions due to widening of use environments of magnetic tapes (in particular, under fluctuating temperature/humidity conditions), reliability on data storage, and reliability on performance, such as stable recording/readout of data in multiple running due to repeating use at high speed, have been increasingly demanded.
Magnetic tapes which are used in digital signal recording systems vary according to each system, for example, magnetic tapes corresponding to a so-called DLT type, 3480, 3490, 3590, QIC, a D8 type and a DDS type are known. In every system, the magnetic tape comprises, on one surface side of a nonmagnetic support, a magnetic layer of a single layer structure having a comparatively thick layer thickness, e.g., from 2.0 to 3.0 .mu.m, containing a ferromagnetic powder, a binder and an abrasive, and a back coating layer provided on the surface side of the support opposite to the side having the magnetic layer for purposes of preventing winding disarrangement and maintaining good running durability. However, in general, in a magnetic layer of a single layer structure having a comparatively thick layer thickness as described above, there is a problem of thickness loss which generates the reduction of output.
For the improvement of the reduction of reproduction output due to thickness loss, thinning of a magnetic layer has been known. For example, JP-A-5-182178 discloses a magnetic recording medium comprising a nonmagnetic support having thereon a lower nonmagnetic layer containing an inorganic powder dispersed in a binder and an upper magnetic layer containing a ferromagnetic powder dispersed in a binder and having a thickness of 1.0 .mu.m or less, which is coated on the lower nonmagnetic layer while the nonmagnetic layer is still wet.
However, with the rapid trend of the increase of the capacity and density of disc-like and tape-like magnetic recording media, it has become difficult to obtain satisfactory characteristics even with these techniques. It has also become difficult to make the increase of the capacity and density compatible with durability.